化学学报 ›› 2013, Vol. 71 ›› Issue (07): 1047-1052.DOI: 10.6023/A13030318 上一篇    下一篇

研究论文

取代基效应对褐煤模型化合物离解焓影响的理论研究

王新华a, 冯莉a, 曹泽星b, 刘祥春a, 汤海燕a, 张曼a   

  1. a 中国矿业大学化工学院 徐州 221116;
    b 厦门大学化学化工学院 厦门 361005
  • 收稿日期:2013-03-21 出版日期:2013-07-14 发布日期:2013-04-23
  • 通讯作者: 冯莉, E-mail: cumthgfl@163.com E-mail:cumthgfl@163.com
  • 基金资助:

    项目受国家重点基础研究发展计划(No. 2012CB214901)和国家自然科学基金(No. 51274197)资助.

Theoretical Study of Substituent Effects on Bond Dissociation Enthalpies in Lignite Model Compounds

Wang Xinhuaa, Feng Lia, Cao Zexingb, Liu Xiangchuna, Tang Haiyana, Zhang Mana   

  1. a School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116;
    b College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005
  • Received:2013-03-21 Online:2013-07-14 Published:2013-04-23
  • Supported by:

    Project supported by the National Basic Research Program of China (No. 2012CB214901) and the National Natural Science Foundation of China (No. 51274197).

为了探究褐煤热解过程中氧桥键C—O均裂这一重要反应, 选取α-O-4和β-O-4类结构单元作为褐煤模型化合物, 运用不同密度泛函计算了部分模型化合物中C—O的离解焓, 并以CBS-QB3作为理论基准值进行比较, 最后选取M05-2X进行离解焓计算. 结果显示, 对于选定的α-O-4和β-O-4类模型化合物, 其平均离解焓分别为51.0 kcal/mol和66.1 kcal/mol. 周围取代环境能显著影响C—O离解焓, 芳环上存在给电子基团(OH, OCH3和CH3)能降低C—O离解焓, 而吸电子基团COOH则能增加其离解焓. 然后深层次分析了取代基效应对C—O离解焓的影响. 此外, 分子内氢键的形成对离解焓也有很大的影响. C—O的离解焓与其键长没有特定的相关性, 不能简单的通过C—O键长来预测其离解焓.

关键词: 密度泛函理论, 离解焓, 褐煤模型化合物, 取代基效应, 分子内氢键

Lignite is an abundant natural resource that is a potential source of clean fuel and value-added chemicals. The mechanisms by which thermal and catalytic treatments deconstruct lignite remain elusive, which is where quantum mechanical calculations can offer fundamental insights. In order to investigate the cleavage of C—O bridge bond, which is the critical step in the thermal decomposition of lignite, the α-O-4 and β-O-4 types of structural units are selected as lignite model compounds to calculate the C—O bond dissociation enthalpies using several kinds of density functional theory methods (B3PW91, B3P86, PBE1PBE, BMK, M06-2X and M05-2X) at 6-31-G(d,p) level. By the comparison between the results and the theoretical benchmark values provided by CBS-QB3 method, M05-2X functional was applied for the calculations on C—O bond dissociation enthalpies. The present results indicate that the C—O average bond dissociation enthalpies are 51.0 kcal/mol and 66.1 kcal/mol for the α-O-4 and β-O-4 types of model compounds, respectively. Local substituents have a great effect on the C—O bond dissociation enthalpies, the C—O bond dissociation enthalpies will decrease when the adjacent arene rings are substituted by electron donating groups (OH, OCH3 and CH3), while the results are opposite for the electron withdrawing groups such as carboxyl group. Then the substituent effects are deeply analyzed on the basis of the ground-state effect and radical effect. An electron donating group can stabilize the phenoxy radicals (radical effect), however, an electron withdrawing group has the opposite effect. In most cases, the radical effect is more important than the ground-state effect. Furthermore, there is a negligible correlation between the C—O bond distances and strengths, and the C—O bond dissociation enthalpies cannot be predicted so easily. Interestingly, the C—O bond dissociation enthalpies can be significantly influenced by the intramolecular hydrogen bond, if the intramolecular hydrogen bond still exists after the cleavage of the C—O bond, the bond dissociation enthalpies will be lower.

Key words: density functional theory, bond dissociation enthalpy, lignite model compounds, substituent effect, intramolecular hydrogen bond